The present invention relates to an antenna for a telecommunications system, in particular a satellite telecommunications system.
Diverse applications often require antennas to receive signals from a mobile source or to transmit signals to a mobile receiver (target). Such transmit and/or receive antennas are usually active antennas made up of immobile radiating elements in which the direction of the radiation pattern can be varied by varying the phase of the signals feeding the radiating elements.
That technique cannot achieve satisfactory radiation patterns for high squint angles, i.e. for directions departing significantly from the mean transmit and/or receive direction.
A source or a receiver can be tracked using motors driving a conventional antenna.
Neither of the above two types of antenna provides a total solution to the problem of communication between the antenna and a plurality of sources or receivers in a large area, in particular an area on the ground, within which communication has to be confined despite the changing position of the antenna relative to the area.
In particular, this problem arises in a telecommunications system using a network of satellites in low Earth orbit. A system of this kind has already been proposed for high bit rate communication between fixed or mobile terrestrial stations within a particular geographical area covering several hundred kilometers. The altitude of the satellites is in the range from 1000 km to 1500 km.
In such systems, each satellite includes groups of receive and transmit antennas and each group is dedicated to a given area on the ground. Within each group, the receive antennas receive the signals from a station in the area and the transmit antennas relay the received signals to another station in the same area. As the satellite moves, the antennas of a group point towards the area at all times so long as the area remains within the field of view of the satellite. Accordingly, for each satellite, a region of the Earth is divided into n areas, and when the satellite moves over a region, a group of transmit and receive antennas is allocated to each area and points toward that area at all times.
In this way, switching from one antenna to another while the satellite is moving over a region, which takes around twenty minutes, for example, and which could be prejudicial to the speed or the quality of communication, is avoided because only one group of transmit and receive antennas is allocated to the area.
Furthermore, the low altitude of the satellites minimizes propagation times, which is favorable to interactive communications, especially for xe2x80x9cmultimediaxe2x80x9d applications.
Clearly, with this telecommunications system, an antenna for one area must not suffer interference from signals from another area and must not interfere with other areas itself.
To solve the above problem of isolating large areas, the invention provides an antenna that can be steered mechanically by drive means and further comprises radiating elements whose radiation pattern is modified as a function of the orientation of the antenna relative to the target or source area to match the pattern to the shape of the target or source area as seen by the antenna.
Accordingly, in the case of the satellite telecommunications system described above, in which the areas are all circular, an antenna on the satellite sees the area as a circle when the satellite is at the nadir of the area. However, as the satellite moves away from that position, the antenna sees the area as an ellipse. The radiating elements, and the control means therefor, which adapt the radiation pattern to the shape of the area as seen by the antenna, then prevent the antenna from receiving signals from other areas or transmitting signals to adjacent areas.
The transmit and receive radiating elements are preferably on a common panel moved by the same drive means.
The pattern is modified by modifying the amplitudes of the signals fed to the radiating elements.
Moreover, in an advantageous embodiment of the invention the radiating elements are disposed on a surface whose shape substantially corresponds to the required radiation pattern for the most distant areas, targets or sources, i.e. the sources supplying the lowest signal levels or the targets to which it is necessary to transmit the maximum power. In other words, the radiating elements adapt to the worst-case scenario.